feat: 实现分层架构，包含 domain、service、repository 和 pkg 层

- 新增 domain 层：model、provider、route、stats 实体
- 新增 service 层：models、providers、routing、stats 业务逻辑
- 新增 repository 层：models、providers、stats 数据访问
- 新增 pkg 工具包：errors、logger、validator
- 新增中间件：CORS、logging、recovery、request ID
- 新增数据库迁移：初始 schema 和索引
- 新增单元测试和集成测试
- 新增规范文档：config-management、database-migration、error-handling、layered-architecture、middleware-system、request-validation、structured-logging、test-coverage
- 移除 config 子包和 model_router（已迁移至分层架构）

backend/internal/service/stats_service_impl.go

@@ -0,0 +1,85 @@
+package service
+
+import (
+	"time"
+
+	"nex/backend/internal/domain"
+	"nex/backend/internal/repository"
+)
+
+type statsService struct {
+	statsRepo repository.StatsRepository
+}
+
+func NewStatsService(statsRepo repository.StatsRepository) StatsService {
+	return &statsService{statsRepo: statsRepo}
+}
+
+func (s *statsService) Record(providerID, modelName string) error {
+	return s.statsRepo.Record(providerID, modelName)
+}
+
+func (s *statsService) Get(providerID, modelName string, startDate, endDate *time.Time) ([]domain.UsageStats, error) {
+	return s.statsRepo.Query(providerID, modelName, startDate, endDate)
+}
+
+func (s *statsService) Aggregate(stats []domain.UsageStats, groupBy string) []map[string]interface{} {
+	switch groupBy {
+	case "provider":
+		return s.aggregateByProvider(stats)
+	case "model":
+		return s.aggregateByModel(stats)
+	case "date":
+		return s.aggregateByDate(stats)
+	default:
+		return s.aggregateByProvider(stats)
+	}
+}
+
+func (s *statsService) aggregateByProvider(stats []domain.UsageStats) []map[string]interface{} {
+	aggregated := make(map[string]int)
+	for _, stat := range stats {
+		aggregated[stat.ProviderID] += stat.RequestCount
+	}
+	result := make([]map[string]interface{}, 0, len(aggregated))
+	for providerID, count := range aggregated {
+		result = append(result, map[string]interface{}{
+			"provider_id":   providerID,
+			"request_count": count,
+		})
+	}
+	return result
+}
+
+func (s *statsService) aggregateByModel(stats []domain.UsageStats) []map[string]interface{} {
+	aggregated := make(map[string]int)
+	for _, stat := range stats {
+		key := stat.ProviderID + "/" + stat.ModelName
+		aggregated[key] += stat.RequestCount
+	}
+	result := make([]map[string]interface{}, 0, len(aggregated))
+	for key, count := range aggregated {
+		result = append(result, map[string]interface{}{
+			"provider_id":   key[:len(key)/2],
+			"model_name":    key[len(key)/2+1:],
+			"request_count": count,
+		})
+	}
+	return result
+}
+
+func (s *statsService) aggregateByDate(stats []domain.UsageStats) []map[string]interface{} {
+	aggregated := make(map[string]int)
+	for _, stat := range stats {
+		key := stat.Date.Format("2006-01-02")
+		aggregated[key] += stat.RequestCount
+	}
+	result := make([]map[string]interface{}, 0, len(aggregated))
+	for date, count := range aggregated {
+		result = append(result, map[string]interface{}{
+			"date":          date,
+			"request_count": count,
+		})
+	}
+	return result
+}